Batteries and electrochemical capacitors represent important systems for energy storage, with applications in electronics, electric vehicles, telephone communication systems, power supplies, and many other applications.
Batteries and electrochemical capacitors based on lithium chemistry have attracted considerable attention as the new generation of energy storage device because of advantages in energy densities and cycle life. During charging, lithium ions are extracted from the positive electrode (cathode) and inserted into the negative electrode (anode) in a lithium ion battery. During discharge, these ions are extracted from the negative electrode (anode) and inserted into the positive electrode (cathode). Materials used for the negative electrode generally include carbonaceous materials, metals, semi-metals, metal oxides, metal sulfides, and metal nitrides. Among them, metal oxides are attractive because of their high specific capacity, good stability in air, and relatively low cost. Among the oxides, iron oxides are particularly interesting because of their high specific capacities (theoretical specific capacity for Fe2O3: 1007 mAh/g), non-toxicity, low cost (price of iron: less than $1/lb in general), and abundance. Electrodes based on iron oxides, however, generally have poor cycling stability resulting from the large volume expansion/contraction during the lithium insertion/extraction cycling process. Moreover, the main lithium extraction potential for iron oxides is relatively high (˜1.7 V vs. Li/Li+), which will limit the output voltage of a full cell. On the other hand, molybdenum oxides are also non-toxic. They may offer better cycling stability and relatively lower lithium-extraction potential than iron oxides. Molybdenum oxides, however, are relatively more costly (price of Mo: over $30/lb in general) and they have a relatively slow lithium insertion/extraction rate limiting their rate capability (M. Stanley Whittingham, 2004). The mixture of iron oxide and molybdenum oxide will be interesting as an electrode material with advantages in good cycling stability, relatively low materials cost, relatively low lithium-extraction potential, and good safety. The presence of iron will lower the materials cost, while molybdenum will improve the mixed materials' cycling stability. Conventional iron-molybdenum mixed oxides have shown poor cycling stabilities, which has limited their applications as electrode materials for energy storage devices.
Therefore, there is a need for developing an iron-molybdenum oxide-based electrode material with good cycling stability. Since the electrode material will be used for commercial devices, it is important that the preparation process can be scaled up with relatively low cost. Therefore, there is also a need for preparing the electrode material by using a wet-chemistry process, which may offer a lower production cost than the conventional chemical vapor deposition and hydrothermal processes that have been used to prepare MoO3 and Fe3O4.